1. Field of the Invention
This invention relates to a vibrator controlling circuit which is used to inform a user of alert (or arrival of a ringing signal) in a portable telephone.
2. Description of the Related Art
In the portable telephone, generally, ringing sound is generated to inform the user of alert. However, the calling sound gives others annoyance during a meeting or within a train. Therefore, recently, a vibrator is vibrated to inform the user of the alert.
Conventionally, the vibrator was vibrated by rotating a motor. However, strong demand of miniaturization and weight reduction has led to demand of vibrating the vibrator without using the motor.
FIG. 9 shows a vibrator controlling circuit for vibrating a vibrator without using a motor. In operation, when a ringing signal is received by an antenna 1, a ringing signal detecting circuit 2 detects the ringing signal to turn on a switch 3. Thus, a power supply voltage VCC is supplied to a signal generating circuit 4. Then, the signal generating circuit 4 starts to operate and generates a square wave signal at a frequency of about 240 Hz to be supplied to the gate of a MOS transistor. When the square wave signal is applied to the MOS transistor 5, the MOS transistor 5 is turned on or off. In this way, the power supply voltage VCC is intermittently supplied to the vibrator 6 to vibrate it, thereby informing a user of alert.
As described above, in the above vibrator controlling circuit, when the ringing signal is received by the antenna, the signal generating circuit generates a signal to turn on or off the MOS transistor 5. Thus, the power supply voltage VCC is intermittently supplied to the vibrator 6 to vibrate it, thereby informing a user of alert. In this case, for example, owing to slippage of a weight attached to the vibrator, if a frequency shift occurs between the resonance frequency of the vibrator and the frequency of the square wave signal generated by the signal generating circuit, as the case may be, the vibration of the vibrator stopped or weakened.